Electric machine, in particular brushless synchronous motor

ABSTRACT

An electrical machine, in particular a brushless synchronous motor has a stator ( 14 ) and a rotor ( 12 ) with salient poles ( 18 ) disposed distributed over its circumference, each of which poles has an enclosed permanent magnet ( 11 ) and a pole shoe ( 19 ) that extends radially outward from the permanent magnet and defines an air gap ( 13 ) between the stator ( 14 ) and rotor ( 12 ). In order to achieve a low cogging torque and a low torque undulation, the pole shoes ( 19 ) are embodied as magnetically anisotropic, with a preferred direction of the greater magnetic conductivity extending parallel to the radial salient pole axis.

BACKGROUND OF THE INVENTION

The invention is based on an electrical machine, in particular abrushless synchronous motor.

In conventional, permanent magnet-excited, brushless synchronous motorsthat are designed as internal rotor motors, the permanent magnets thatgenerate the excitation field in the working air gap between the rotorand stator are embodied in the form of shell segments affixed to thesurface of the rotor. If it is optimized, a synchronous motor of thiskind has a low cogging torque and a low degree of torque undulation,which is very favorable for certain uses of the motor in which a verysmooth torque is required. The disadvantage of this motor is the highproduction cost, due in particular to the production of the shell-shapedpermanent magnets, which must be ground in order to produce the shellshape.

In order to reduce the production costs, brushless synchronous motorshave therefore been developed in which the permanent magnets embedded inthe rotor are “buried” in it (EP 1 028 047 A2). The “buried” permanentmagnets are mostly rectangular and can be easily cut from a large blockof permanent magnet material, which reduces costs in comparison to ashell-segment magnet. Even when optimized, such a brushless synchronousmotor with “internally buried” permanent magnets, also referred to as anIPM motor, tends to have a significantly higher cogging torque andhigher degree of torque undulation than a comparable motor withshell-shaped permanent magnet segments on the surface of the rotor.

SUMMARY OF THE INVENTION

The electrical machine according to the invention, in particular thebrushless synchronous motor according to the invention with the featuresof claim 1, has the advantage over the known IPM motor of asignificantly reduced cogging torque and a significantly lower degree oftorque undulation. The manufacturing costs are on a par with those ofthe known IPM motor and are significantly lower than those of the knownbrushless synchronous motor with shell-shaped permanent magnet segmentson the surface of the rotor.

As opposed to the known IPM motors that have an approximatelytrapezoidal voltage curve, the magnetic anisotropy of the pole shoesaccording to the invention, i.e. of the regions of the rotor disposed infront of the “buried” permanent magnets in the direction toward theworking air gap, results in a sinusoidal curve of the induced voltage orEMF, so that the harmonic distortion of the EMF is lower, thus reducingthe torque undulation when sinusoidal supply current is used. The peakvalue of the EMF increases and therefore so does the average torqueproduced by the motor, when no prior commutation is used. In addition,the magnetic anisotropy of the pole shoes reduces the iron losses in thepole shoes and also reduces the transverse field of the armature. Forpowerful winding currents, the “salience” (i.e. the ratio of shuntinductance L_(q) to series inductance L_(d)), which should ideally be 1,is reduced in comparison to that in the known IPM motors, which likewisereduces torque undulation.

According to a preferred embodiment of the invention, the permanentmagnets are embodied in a rectangular form and have a magnetizationdirection extending parallel to the normals of the larger magnetsurfaces. The preferred direction of the greater magnetic conductivityor relative permeability of the pole shoes is aligned so that it pointsin the magnetization direction of the permanent magnets.

According to an advantageous embodiment of the invention, in order toachieve the magnetic anisotropy, a multitude of flux barriers spacedapart from one another are incorporated into the pole shoes, extendingparallel to the radial salient pole axis. Preferably the flux barriersare comprised of cutouts in the pole shoes, but can alternatively alsobe embodied in the form of inclusions made of magnetically nonconductivematerial. The embodiment of the flux barriers in the form of cutouts hasthe advantage of reducing the mass of the pole shoes, for example areduction of approx. 34% for a six-pole or four-pole embodiment of therotor. The reduced mass results in a reduction in the moment of inertia,which in turn improves the dynamic properties of the electrical machine.The reduced centrifugal force permits a higher speed to be achieved withthe same radial strut width.

According to an alternative embodiment of the invention, in order toachieve the magnetic anisotropy, the pole shoes are made of2D-anisotropic SMC (soft magnetic composite)-powdered iron material. Themagnetic property of this material is approximately four to five timesgreater in the preferred direction than perpendicular to the preferreddirection; for example the relative permeability in the preferreddirection is approximately 800 and the relative permeabilityperpendicular to the preferred direction is approximately 200.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in detail in the description below, inconjunction with exemplary embodiments shown in the drawings.

FIG. 1 shows a schematic cross section through a six-pole brushlesssynchronous motor,

FIG. 2 is an enlarged depiction of the detail II in FIG. 1, with fluxpaths indicated,

FIGS. 3 to 9 show various graphs of the properties of the motor toillustrate the advantages over the known IPM motor,

FIG. 10 schematically depicts a cross sectional detail of a brushlesssynchronous motor according to another exemplary embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The brushless synchronous motor, a cross section of which isschematically depicted in FIG. 1 as an exemplary embodiment of a genericelectrical machine, is embodied as an internal rotor motor and has arotor 12 equipped with permanent magnets 11 and a stator 14 thatconcentrically encompasses the rotor 12 to form a working air gap 13.The stator 14 is comprised of a yoke ring 15 and a multitude of statorteeth 16, which protrude radially inward from the yoke ring 15 and aredisposed equidistantly from one another in the circumference direction.Each stator tooth 16 has an annular coil 17 wound onto it; FIG. 1depicts the annular coils 17 of only one phase or winding of the statorwinding. In the exemplary embodiment in FIG. 1, the stator winding isembodied as a three-phase or three-winding stator, with three annularcoils 17 connected in series or in parallel. There are therefore ninestator teeth 16.

The rotor of the for example six-pole synchronous motor has six salientpoles 18, each with an enclosed permanent magnet 11 and a radiallyadjoining pole shoe 19 extending outward from it toward the air gap 13.The rotor 12 has a polygonal, prismatic rotor core 20, which in theexemplary embodiment of the six-pole synchronous motor, is a hexagonalprism. A central, cylindrical opening 26 allows the rotor core 20 to beslid onto a rotor shaft. Each prism side of the hexagonal rotor core 20is adjoined by the large magnetic surface 111 of a flat, block-shapedpermanent magnet 11 (FIG. 2) so that the normal on the magnet surface isaligned in the radial direction. The identically sized magnet surface112 oriented away from this magnet surface 111 is covered by arespective pole shoe 19. The pole shoes 19 of all of the salient poles18 abut one another in the circumference direction, are of one piecewith one another, and are also of one piece with the rotor core 20,fastened to it by means of narrow radial struts 21; the radial struts 21protrude outward from the vertices of the hexagonal prism. The rotorcore 20 with the pole shoes 19 and radial struts 21 is comprised of amultitude of one-piece profiled sheet metal plates stacked against oneanother. The permanent magnets 11 contained inside the salient poles 18are magnetized so that their magnetization direction extends parallel tothe normals on the large magnet surfaces 111, 112; viewed in thecircumference direction, the magnetization direction in succeedingpermanent magnets 11 is rotated by 180° so that the magnetizationdirections in neighboring salient poles 18 are inverse to each other.

In order to produce a sinusoidal curve of the induced voltage, whichalong with a sinusoidal winding current, produces a torque curve with avery low degree of torque undulation, the pole shoes 19 disposed on theside the permanent magnets 11 oriented toward the air gap 13 areembodied as magnetically anisotropic; they have a preferred direction ofgreater magnetic conductivity or permeability that extends parallel tothe radial salient pole axis, thus coinciding with the magnetizationdirection of the permanent magnets 11. This anisotropy of the salientpoles 18 forces the magnetic flux in the salient poles 18 to travel in aparallel direction, as can be seen in the course of the flux linesdepicted in FIG. 2.

In the exemplary embodiment of the synchronous motor depicted in FIGS. 1and 2, the magnetic anisotropy is achieved by means of a number of fluxbarriers 22 that are incorporated into the pole shoes 19 spaced apartfrom one another so that they extend parallel to the radial salient poleaxis. The flux barriers 22 for the magnetic flux are embodied as cutouts23, which, together with the sheet metal cutting of the sheet metalplates for the rotor 12, are stamped out in comb-like fashion, with thecomb openings oriented toward the sheet metal core, which when thepermanent magnets 11 are then inserted, rest against their magnetsurfaces 112. The number of cutouts 23 per pole shoe 19 depends on thewidth of the salient poles 18, i.e. the width of the permanent magnets11 and pole shoes 19 viewed in the circumference direction, and thethickness of the sheet metal plates. Preferably, as large as possible anumber of cutouts 23 is provided (approx. ten to twenty in smallmotors); the number of cutouts 23 on the circumference increases alongwith the number of poles of the synchronous motor.

As can be seen from the drawings, the flux barriers 22 are arranged oneafter the other and distributed over the whole width b of each of thepermanent magnets 11 and also laterally beyond both peripheral ends ofeach of the permanent magnets 11 as considered in a direction of thewidth b of each of the permanent magnets 11.

In a manner that is not shown here, the flux barriers can alternativelyalso be comprised of inclusions made of magnetically nonconductivematerial; the inclusions are once again disposed parallel to and spacedapart from one another in the pole shoes 19.

The graphs in FIGS. 3 to 9 depict the advantageous electromagneticproperties of the novel synchronous motor described above in conjunctionwith FIGS. 1 and 2 in comparison to a known, similarly designed IPMmotor without flux barriers. The curves labeled “1” in the graphs depictthe synchronous motor with flux barriers according to FIGS. 1 and 2, thecurves labeled “2” belong to the known, conventional IPM motor withoutflux barriers in the pole shoes. The different electromagneticproperties of the synchronous motors according to the graphs in FIGS. 3to 5 and FIGS. 7 and 8 are each depicted as a function of the magnetwidth b as defined in FIG. 2. The magnet width b is indicated in angularelectric degrees. The results depicted relate to motors without inclinedrotors or stators.

The graph in FIG. 3 shows the cogging torque curve as a function of themagnet width b. The optimal angle of the magnet width b of thesynchronous motor with flux barriers is approximately 128 electricdegrees, at which a peak-to-peak value of the cogging torque of only0.05 Nm is achieved, in comparison to an optimal value of 0.57 Nm forthe known IPM motor at an optimal magnet width angle of 120 electricdegrees. The anisotropic salient poles 18 with the multiple parallelflux barriers 22 thus achieve a 91% reduction in the minimalpeak-to-peak value of the cogging torque. It should be noted that thisachieved value of 0.05 Nm is also better than the minimal value that isachieved by a comparable motor with shell magnets on the surface of therotor 12.

The graph in FIG. 4 depicts the harmonic distortion of the inducedvoltage curve as a function of the magnet width b. At 140° of magnetwidth, a harmonic distortion of the voltage curve of 1% is produced, incomparison to the optimal value of 2.2% for the conventional IPM motorwithout anisotropy at an optimal magnet width angle of 127 electricdegrees. This harmonic distortion of 1.0% is identical to the minimalvalue that is achieved by a comparable motor with shell segments on thesurface of the rotor.

FIG. 5 shows the peak values of the first harmonic of the curve of theinduced voltage (electromotive force EMF) over the magnet width b. Thepeak of the first harmonic value of the synchronous motor with fluxbarriers is an average of 1.2% higher than in the known IPM motors atall magnet widths. The maximal increase in the peak of the firstharmonic is 8.5% higher than in the known IPM motor at a magnet width of120 electric degrees.

FIG. 6 shows the curve of the induced voltage (EMF) of one winding ofthe synchronous motor with flux barriers in comparison to the known IPMmotor without flux barriers, for an equal magnet width of 140 electricdegrees. When the harmonic distortion of only 1.0% produced by thesynchronous motor with flux barriers is compared to 3.9% in the knownIPM motor (see FIG. 4), the curve of the EMF is distinctly moresinusoidal than in the known IPM motor, which has a more trapezoidalcurve.

FIG. 7 shows the torque undulation of the two motors to be compared, atdifferent magnet widths b. The optimal value of the synchronous motorwith flux barriers is approximately 133 electric degrees; a peak-to-peakvalue of the torque undulation of only 0.34 Nm is achieved as comparedto an optimal value of 0.48 Nm for the known IPM motor at an optimalmagnet width angle of 120 electric degrees. In the synchronous motoraccording to the invention, the use of anisotropic salient pole geometrywith multiple parallel flux barriers achieves a 29% reduction in thepeak-to-peak value of the torque undulation. It should be noted thatthis undulation of 0.34 Nm is also better than the minimal value that isachieved in a comparable motor with shell segments on the surface of therotor 12.

FIG. 8 shows the average torque as a function of the magnet width b. Theaverage torque of the synchronous motor with flux barriers is an averageof 3.1% higher than in the known IPM motor at all magnet widths. Themaximal increase in the average torque is 4.8%. No prior commutation wasused.

Graph 9 depicts the salience L_(q)/L_(d) of the two motors as a functionof the effective value of the winding current. For a use of thesynchronous motor in which a very low torque undulation is required, themotor should have a salience of 1.0% over the entire winding currentspectrum. This requirement is fulfilled by motors with shell magnets onthe surface of the rotor. If the shunt inductance L_(q) is greater thanthe series inductance L_(d), then this can lead to an increase in thetorque undulation.

In the known IPM motor without flux barriers, as the winding currentincreases, the longitudinal axis flux path becomes saturated, as aresult of which the series inductance L_(d) decreases. The transverseaxis flux path, however, is not saturated and therefore the shuntinductance L_(q) remains virtually constant when there is an increase inthe winding current. Because of this behavior, the salience of the knownIPM motor increases with the winding current (see curve 2 in FIG. 9). Inthis example, the maximal salience of the known IPM motor is 1.45 at 40A of effective winding current. In the synchronous motor with fluxbarriers according to the invention, with an increase in the windingcurrent, the longitudinal axis and transverse axis flux paths becomesaturated. The shunt inductance decrease, however, is less than theseries inductance decrease. The salience of 1.0 is not in fact achieved,but the salience is sharply reduced in comparison to the known IPM motorand in this example, is only 1.15 at 15 A of effective winding current.

In the modified synchronous motor depicted in the cross sectional detailin FIG. 10, in order to achieve the magnetic anisotropy in the salientpoles 18, the pole shoes are made of 2D-anisotropic SMC (soft magneticcomposite)-powdered iron material. This material has a preferreddirection of the magnetization or magnetic permeability, as indicated bythe arrow 24 in FIG. 10. The magnetic properties of the material in thismagnetic preferred direction are significantly greater, in this instanceapproximately four to five times greater than the magnetic properties ofthis material perpendicular to the preferred direction. For example, therelative permeability of the material in the preferred direction (arrow24) is approximately 800, while the maximal relative permeabilitylateral to the preferred direction (perpendicular to arrow 24) isapproximately 200. This core-oriented SMC-powdered iron material alsoachieves a parallel flux course in the pole shoes 19 in the same manneras the provision of flux barriers, which results in the above-describedadvantageous properties of the motor. The magnetization direction 27 ofthe permanent magnet 11 is symbolized by the arrow 27 in FIG. 10. Themagnetization directions 27 in the two adjacent permanent magnets 11 areinverse to each other.

In the rotor according to FIG. 10 as well, the rotor core 20 is embodiedas a polygonal prism and with a six-pole design of the synchronousmotor—as shown, is embodied in the form of a hexagonal prism. Thelikewise block-shaped permanent magnets 11 each rest against one surfaceof the hexagonal prism and on their sides oriented away from the prism,are each covered by a pole shoe 19. The pole shoes 19 are encompassed bya concentric protective tube 25 made of a material that is magneticallynonconductive or has a low magnetic conductivity, which protects the SMCmaterial of the pole shoes 19 and the rotor 12 as a whole. The rotorcore 20 is comprised of solid steel or is instead comprised of amultitude of sheet metal plates stacked against one another, that eachhave a hexagonal profile with a central, circular opening 26 stamped outfrom them.

1. An electrical machine, in particular a brushless synchronous motorthat has a stator (14) and a rotor (12) with salient poles (18) disposeddistributed over its circumference, each of which poles has an enclosedpermanent magnet (11) and a pole shoe (19) that extends radially outwardfrom the permanent magnet and defines an air gap (13) between the stator(14) and rotor (12), characterized in that the pole shoes (19) areembodied as magnetically anisotropic, with a preferred direction of thegreater magnetic conductivity extending parallel to the radial salientpole axis, wherein in order to achieve the magnetic anisotropy, amultitude of flux barriers (22) that are spaced apart from one anotherare incorporated into the pole shoes (19), extending parallel to theradial salient pole axis, wherein the flux barriers (22) are comprisedof cutouts in the pole shoes (19), wherein the rotor (12) has apolygonal, prismatic rotor core (20), which the permanent magnets (11)rest against, wherein the pole shoes (19) cover the permanent magnets(11) on their sides oriented away from the rotor core (20), abut oneanother in one piece in the circumference direction, and at theseabutting locations, are fastened to the rotor core (20) in one piece bymeans of narrow radial struts (21), wherein the rotor core (20) with thepole shoes (19) and radial struts (21) is comprised of a multitude ofone-piece sheet metal plates stacked against one another, and whereinthe cutouts (23) are stamped out in comb-like fashion, with combopenings in the sheet metal plates oriented toward insertion openingsfor the permanent magnets (11), and wherein a width of the flux barriers(22) is greater than a width b of the permanent magnets (11), andwherein the flux barriers (22) are arranged one after the other anddistributed over the whole width b of each of the permanent magnets (11)and also laterally beyond both peripheral ends of each of the permanentmagnets (11) as considered in a direction of the width b of each of thepermanent magnets (11).
 2. The machine according to claim 1, wherein theflux barriers (22) are comprised of inclusions made of magneticallynonconductive material.
 3. The machine according to claim 1, wherein thenumber of cutouts (23) per pole shoe (19) depends on the width of thesalient pole and the thickness of the sheet metal plates.
 4. The machineaccording to claim 1, wherein the permanent magnets (11) are embodied asblock-shaped and have a magnetization direction extending parallel tothe normals on the largest magnet surfaces (111, 112) and that thepreferred direction of the greatest magnetic conductivity of the poleshoes (19) coincides with the magnetization direction (27) of thepermanent magnets (11).